A resonator fiber optic gyroscope (RFOG) is a sensing apparatus that senses rotation rates by measuring rotation induced resonance frequency difference of the RFOG ring cavity in two counter-propagating directions (referred herein as clockwise (CW) and counter-clockwise (CCW) directions). In order to measure the ring cavity resonance frequency difference, monochromatic lightwaves from separately tunable lasers are typically phase modulated and then coupled into the RFOG ring cavity in CW and CCW directions. A fraction of the circulating lightwaves are coupled out of the ring cavity and directed to photodetectors to generate photocurrent (or voltage) signals. Demodulation of the photodetector signal at the corresponding phase modulation frequencies generates discriminant signals for detection of the resonance frequency difference of CW and CCW directions.
Ideally, CW and CCW propagating lightwaves are received by separate detectors (referred to herein as CW and CCW detectors, respectively) and no cross-coupling happens, that is, no CW (CCW) lightwaves are received by the CCW (CW) detector. In practical situations, however, there are imperfect optical surfaces in the ring cavity that may weakly reflect lightwaves into the counter-propagating direction, causing fractions of the CW (CCW) beam to be received by the CCW (CW) photodetector. This back reflection is often categorized as of “single-back-reflection” type, because it is characterized by directing lightwaves onto wrong detectors through one (or odd number of) reflection(s). Using different phase modulation frequencies for the CW and CCW resonance detection is an effective way to isolate the erroneous reflected signal.
In the case when CW and CCW lightwaves have nearly the same optical frequency, the interference beat signal falls within the gyro bandwidth, causing signal fluctuations that can significantly degrade the gyro performance. To solve this problem, schemes have been previously suggested to use multiple laser beams in the CW and CCW direction in such a way that their frequencies are separated by integer numbers of resonator free spectral range (FSR), which is the frequency separation between neighboring resonances, typically, a few to a few hundred MHz. Since the beat noise frequencies are in the high frequency region, such frequencies can be filtered out by low-pass electronic filters.
There is another type of reflection called “double-back-reflection” (or simply “double reflection”) whose impact on the stability of rotation rate measurement (also called rotation rate stability) cannot be easily removed by the above mentioned methods. Double reflections cause lightwaves to interfere with the reflected portion of themselves produced by two (or even number) of reflections in the optical path. For this type of back reflection, the interfering lightwaves are originated from the same laser, having the same optical frequency and the same phase modulation. The interfering lightwaves cannot be easily isolated from the main signal beam in the demodulation process. The erroneous lightwaves, which are turned into a counter-propagating direction through a first reflection point and then turned back into the original direction by a second reflection point, propagate through a different optical path as the main beam. Environmentally induced path length variation causes the relative phase changes between the two interfering beams, leading to rotation rate instability of the gyroscope. This can be a significant factor that degrades RFOG performance.